When using virtual hostnames and someone requests the IP instead of a hostname, Apache might use the ServerName setting to redirect clients to the correct URL.

Therefore, you should define the ServerName value. On Debian-based Apache 2 installations, you can do this in the file "/etc/apache2/httpd.conf", which should be empty by default. Just add

ServerName my.website.com

If you're only having Apache as development server, you can simply ignore the warning. Apache will use reverse DNS to lookup for requests to 127.0.0.1, for example, and thus automatically find out that the hostname is "localhost".

Another way is to edit the "/etc/hosts" file. That will work as well because of named reverse DNS lookup.